An active matrix substrate for use in liquid crystal display devices, or the like, includes a switching element, such as a thin film transistor (hereinafter, “TFT”), in each pixel. Examples of such a switching element which have been conventionally used in various applications include a TFT which includes an amorphous silicon film as the active layer (hereinafter, “amorphous silicon TFT”) and a TFT which includes a polycrystalline silicon film as the active layer (hereinafter, “polycrystalline silicon TFT”).
In recent years, using materials other than amorphous silicon and polycrystalline silicon as the material of the active layer of TFTs has been attempted. For example, Patent Document 1 discloses a liquid crystal display device in which the active layer of TFTs is formed using an oxide semiconductor film of InGaZnO (an oxide consisting of indium, gallium and zinc), or the like. Such a TFT is referred to as “oxide semiconductor TFT”.
The oxide semiconductor TFT is capable of operating at a higher speed than the amorphous silicon TFT. The oxide semiconductor film is manufactured through a simpler process than the polycrystalline silicon film and is therefore applicable to devices which require a large area. Therefore, application of the oxide semiconductor TFT, as a high-performance active element which can be manufactured with reduced manufacturing steps and a reduced manufacturing cost, to display devices, and the like, has been encouraged.
Since the mobility of the oxide semiconductor is high, it is possible to achieve equal or higher performance even if the size is reduced as compared with conventional amorphous silicon TFTs. Therefore, when an active matrix substrate of a display device is manufactured using the oxide semiconductor TFT, the area occupancy of the TFT in each pixel can be reduced, and the pixel aperture ratio can be improved. Accordingly, even when the amount of light from the backlight is reduced, bright display can be realized, and low power consumption can be realized.
Particularly in small-size, high-resolution display devices for use in smartphones and the like, it is difficult to increase the pixel aperture ratio due to, for example, limitations on the minimum width of wires (process rules). In view of such, if the pixel aperture ratio is improved by using the oxide semiconductor TFT, display of high-resolution images can be realized while the power consumption is reduced even in small-size display devices.
Since the off-leak characteristic of the oxide semiconductor TFT is excellent, it is possible to utilize an operation mode where display is performed with reduced image rewriting frequency. For example, in the case of displaying a still image, it is possible to operate the TFT so as to rewrite the image data once every second. Such a driving method is referred to as “intermittent driving” or “low frequency driving” and is capable of greatly reducing the power consumption by the display device.